If you can no longer run, then power walk, speedwalk, or racewalk. Your legs need some impact exercise, and cycling alone will lead to osteoporosis. It is very common for elite cyclists in their 40s to have the hearts of 20-year-olds and the bones of 60-year-olds.
I still run on dirt or sand, and occasionally on asphalt, but I have had to give up running on concrete -- too hard on the ankles and feet, although my knees can still handle it. My fitness program includes weight training, running, walking, and cycling, and the latter three serve many of my transportation needs, as well.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069